The present invention relates to a scanning optical system employed, for example, in a color laser beam printer, color laser copier or the like.
Color printers have been developed and widely used for an output device of computers, color copiers or the like. In order to form a color image at a high speed, a color laser printer is widely employed. The color laser beam printer typically includes a plurality of photoconductive drums, and corona chargers, developing units etc. for respective color components, i.e., yellow (Y), magenta (M), cyan (C) and black (B).
In order to form electrostatic latent images consisting of the four color components on the photoconductive drums, respectively, four scanning optical systems, each of which has a polygonal mirror and imaging optical system, may be used. However, such an apparatus provided with four polygonal mirrors and four imaging optical systems for four photoconductive drums costs a lot in manufacturing thereof. Recently, an apparatus making use of a single polygonal mirror for simultaneously deflecting four beams corresponding to the four color components has been developed. The four deflected beams are incident on four imaging optical systems, respectively, thereby the four beams being directed to the four photoconductive drums. Thus, four image components can be formed simultaneously.
In order to simultaneously deflect a plurality of laser beams using a single polygonal mirror, in particular, to make a plurality of beams be incident on the plurality of imaging optical systems arranged separately in the auxiliary scanning direction, respectively, the beams should be inclined at different angles with respect to a plane perpendicular to a rotational axis of the polygonal mirror, or the beams are separated from each other in the auxiliary scanning direction.
If laser beams are inclined with respect to a plane perpendicular to the rotation axis of the polygonal mirror, the scanning lines, which are loci formed by moving beam spots on the surfaces to be scanned, curve in the auxiliary scanning direction (i.e., in a circumferential direction of photoconductive drums). By appropriately arranging a lens, which is one of the lenses of the imaging lens, having a power to converge a beam in the auxiliary scanning direction and located closer to the surface to be scanned than the other lenses, the curvature of the scanning lines in the auxiliary scanning direction, or the bow can be suppressed by a certain degree. However, the above-described lens should be manufactured to have a relatively wide effective area. Further, in view of the aberration compensation, it is preferable that the refraction surfaces have asymmetrical surface. Therefore, manufacturing such a lens is very difficult. Further, four of such lenses should be prepared, which increases the manufacturing cost of the entire scanning optical system.
Further, there is a requirement, among manufacturers, that if a certain accuracy is to be maintained the shape of the lenses should be the same to suppress the manufacturing cost.